There have been various known techniques of restraining occupants of motorcycles with airbag apparatuses. For example, known techniques include a technique of restraining an occupant with an airbag inflated by inflation gas in a frontal collision (e.g., refer to Japanese Unexamined Patent Application Publication No. 2003-327182). The airbag apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2003-327182 has a structure in which a prefolded airbag is housed in a bottomed-box-shaped housing and in which an airbag opening arranged at the top of the housing is covered with a cover (airbag cover). The cover is torn open along a fragile portion with the deploying motion of the airbag to open the airbag opening, thereby enabling the deploying motion of the airbag from the interior of the housing to the exterior.
The housing for accommodating a prefolded airbag, as in the airbag apparatus described in Japanese Unexamined Patent Application Publication No. 2003-327182, has a bottomed box shape having an airbag opening at the top. Accordingly, it has the problem that even if the tightness of the housing is increased, liquid such as water or oil may be prone to enter or reside in the housing through the periphery of the airbag opening. Particularly, general motorcycles have an airbag apparatus exposed to the outside of the vehicle body in contrast to cars, so that such a problem may be pronounced under the influence of rain, cleaning water, and humidity.